1. Field of the Invention
This invention relates to a process for the preparation of vanadyl alcoholates, particularly vanadyl alcoholates of the formula: O = V(OR).sub.3 (Y).sub.n wherein Y is .tbd.V= O and n is 0 or 1 and when n is O, R is alkyl, cycloalkyl, alkylaryl, arylalkyl, aryl, alkoxyaryl or hydroxyalkyl and when n is 1, R is an alkoxy radical. This invention is particularly addressed to the problem of removing water formed during the preparation of vanadyl alcoholates by reaction of vanadyl pentoxide with an alcohol. This invention is also directed to the preparation of vanadyl alcoholates of high purity.
2. Description of the Prior Art
It is known that vanadium pentoxide can be reacted with alcohols to form the corresponding vanadyl alcoholates according to the following reaction: EQU V.sub.2 O.sub.5 + 6 ROH .revreaction.2 VO(OR).sub.3 + 3H.sub.2 O
since this reaction is an equilibrium reaction it is necessary, in order to favor the formation of esters, to continually remove the water that forms. The continual removal of the water, however, involves great difficulty.
It is known to remove the water by distilling over the water as it forms in the reaction, together with excess alcohol, into a second reaction vessel containing a substance which absorbs water, such as quicklime for example. At the boiling temperature of the alcohol, and with intensive stirring, the alcohol is dewatered in the second reaction vessel and then distilled back into the first reaction vessel in which the reaction with vanadium pentoxide is taking place. (Cf. German Offenlegungschrift 1,816,386).
This known process, however, is very expensive. For example, the water-removing substance must be constantly replaced or regenerated. Also, constant vigilance is necessary during the reaction to prevent the reaction from stopping prematurely due to the exhaustion of the water-removing substance. Heat must continually be introduced into the mixture captured in the second reaction vessel in order to keep it boiling. This relatively great energy consumption is particularly appreciable when operating on a commercial scale.
Another disadvantage of this known process consists in the fact that it is necessary to operate in the presence of a strongly acid catalyst. Despite the presence of the catalysts, however, it is necessary to reflux the reaction mixture for more than 8 hours as a rule if it is desired to achieve pure ester yields of about 50%. Yields greater that 50% can be achieved only when operating on a laboratory scale. When the procedure is used on a commercial scale it is found that the yields vary from batch to batch. Consequently, the prior-art process does not achieve reproducible results to a sufficient extent.
Operating in the presence of strongly acid catalysts results in additional disadvantages. The amounts of acid added accelerate the reaction of the V.sub.2 O.sub.5 with the alcohol to form the ester, and yet they promote the reduction of the pentavalent vanadium to tetravalent vanadium with the cooperation of the corresponding alcohol. The side reaction, however, is not desired. Since the compounds of the strongly acid reaction are used in practice preferably in amounts of 1 to 10 weight percent with reference to the input vanadium, they are not present in only catalytically effective amounts. Rather, they are present in appreciable percentages which constitutes an impurity in the reaction mixture product. Thus, sulfuric acid is usually found in the form of vanadyl sulfate, in which the vanadium is in the oxidative tetravalent stage. Thus, for example, in a solution of vanadium oxytriisopropylate in isopropanol, with a moisture content of 0.2 weight percent the main part of the tetravalent vanadium is found in the form of solid vanadyl sulfate in the unreacted vanadium pentoxide. In the case of higher moisture contents, the vanadyl sulfate is contained increasingly in the reaction mixture and interferes with the processing.
For example, out of 200 g of V.sub.2 O.sub.5, 3000 ml. of isopropanol and 10 ml. of concentrated sulfuric acid, one obtains after 3 hours of reaction solution which still contains 0.39 weight percent of water. When the unreacted V.sub.2 O.sub.5 is filtered out of the solution and the isopropanol has been removed by evaporation and the ester has been vacuum distilled, 143 g (= 26% yield) of pure vanadium oxytriisopropylate is obtained. An extraordinarily great quantity of 109 g of distillation residue remains as an unwanted by-product probably due to the high percentage of sulfuric acid and the excessively great moisture content of the reaction solution.
If organic sulfonic acids, such as toluenesulfonic and benzenesulfonic acid, are used instead of sulfuric acid, similar phenomena occur. In the preparation of vanadium oxytriisopropylate, voluminous greenish flakes precipitate from the greenish reaction solution during the progressive concentration and the distillative refinement of the ester that follows; these flakes interfere with the distillation and increase the percentage of distillation residue.
Attempts have already been made to use phenol derivatives or weak acids such as boric acid, for example, as catalysts instead of the strong acids. In the reaction of alcohols having up to 4 carbon atoms, however, these catalysts display no activity (French Pat. 1,271,641).
It is also known in the preparation of vanadyl alcoholates to reflux stoichiometric amounts of vanadium pentoxide and an alcohol having 5 to 5 carbons, respectively, in the presence of benzene (with a ratio of alcohol to benzene of 1 : 1.1 to 1 : 1.35 by volume). The water formed at the boiling temperature is removed from the reaction zone as an azeotropic mixture by the addition of benzene and can then be separated as the heavy phase in a water separator.
At a reaction time of 8 to 12 hours the yields amount to only between 10 and 32% with reference to the vanadium pentoxide input. Especially in the preparation of vanadium oxitri-n-butylate, yields of only 26%, for example, have been achieved (cf. Bull. Acad. Sci. USSR 1957, pages 899-900).
In the preparation of vanadium oxitri-n-butylate it has also been proposed to add toluene instead of benzene to the reaction mixture as an extractant in order to increase the yield. Yields of up to about 65% can be achieved by this process (cf. U.S. Pat. No. 3,657,295), but the time required for the reaction is very long, amounting to as much as 24 hours. In this process a mixture of vanadium pentoxide, toluene and n-butanol is heated to ebullition and the water forming in the reaction is distilled together with the toluene as an azeotropic mixture into a water separator. Here a water-rich phase and a hydrocarbon-rich phase are formed. The latter is continuously recycled to the reactor and the water-rich phase is separated at intervals. Using 1.3 to 1.4 times the stoichiometrically required amount of alcohol, the highest yields are achieved in this case, of 65%. According to this patent, the use of a greater excess of alcohol will not achieve any technical effect as regards increasing the yield, increasing the reaction speed, or the like.
The long reaction time (24 hr.) that is required if it is desired to achieve yields of 65% is particularly disadvantageous. Another disadvantage is that very large amounts of toluene must be used (the ratio of toluene to n-butanol is preferably to be from 1 : 1 to 3 : 1 by volume). In the processing of the reaction mixture that follows, the excess solvent -- in this case unreacted alcohol together with toluene -- is removed by distillation. It has been found disadvantageous that, before the distillate can be used for the next batch, it must first be readjusted to a specific toluene-butanol content, because otherwise reproducible results cannot be achieved. For this purpose, however, complicated procedures are necessary, which make this process uneconomical and render technical scale operation difficult.
To avoid these difficulties in the prior art methods based on the reaction of V.sub.2 O.sub.5 with alcohols it has also been proposed that vanadyl alcoholates be prepared by starting out with VOCl.sub.3 and reacting it with alkali metal alcoholates or alcohols; this results in yields of 60% (with reference to VOCl.sub.3). These known methods, however, have the disadvantage that the reactant VOCl.sub.3, which is very sensitive to hydrolysis, has to be prepared in a separate procedure from V.sub.2 O.sub.5. Furthermore, undesired vanadium-containing by-products form in this process, as well as alkali chlorides or HCl as reaction products. The hydrochloric acid that forms has to be neutralized with ammonia in an another separate procedure, so that, by and large, the process is a very complicated one. In addition, these prior-art methods result in products which are not entirely chloride-free. The esters prepared in this manner usually have a reduced shelf life, which is indicated by a dark discoloration. Often they are then no longer suitable for use as a component of a polymerization catalyst.
The preparation of vanadyl alcoholates of C.sub.2 to C.sub.4 alcohols by the transesterification of a lower vanadyl alcoholate with a correspondingly higher boiling alcohol is relatively difficult. These transesterification methods require that the starting product be a vanadyl alcoholate of a low alcohol which in turn is supposed to be more easily accessible than the desired vanadyl alcoholate of a higher alcohol. For the preparation of such "low" alcoholates, however, a process like the one described above (on the basis of VOCl.sub.3) has hitherto been recommended. In the transesterification processes, losses of yield have always had to be accepted in order to prepare pure esters, so that the problem of manufacturing pure esters with a high space-time yield, by the use of a very simple method, has basically not yet been solved.
It is, therefore, an object of the present invention to provide a process for the preparation of vanadyl alcoholates starting with vanadyl pentoxide. It is a particular object of the present invention to prepare vanadyl alcoholates in high purity in good yields within a commercially feasible period of time and especially without the use of separate reaction zones, recycling procedures and the like. It is a particular object of the present invention to provide a process by which those vanadyl alcoholates which have heretofore proved difficult to prepare can be synthesized in high purity and within commercially feasible period of time with respect to space-time yield.